Enkephalin, a natural opiate receptor agonist in the brain, has been identified [see Hughes et al., Nature, 256, 577 (1975)] as a mixture of two pentapeptides: H-Tyr-Gly-Gly-Phe-Met-OH (methionine-enkephalin) and H-Tyr-Gly-Gly-Phe-Leu-OH (leucine-enkephalin). Both peptides mimic the ability of morphine to block electrically evoked contractions of mouse vas deferens and guinea pig ileum, and both inhibit the stereospecific receptor binding of the opiate antagonist 3H-naloxone in brain homogenates.
It has been proposed that enkephalin receptors may be sites at which morphine-like drugs exert their analgesic activities, and that enkephalin may be the modulator or transmitter in brain systems for pain suppression or analgesia. It has been reported that methionine-enkephalin and leucine-enkephalin, when administered by injection in the brain ventricle in rats, induce a profound analgesia that is fully reversible by naloxone, [see Belluzzi et al., Nature, 260, 625 (1976)]. The enkephalins are inactive when administered peripherally, however, and it is believed that the enkephalins are rapidly destroyed by blood enzymes and/or are poorly transported across the blood-brain barrier.
The amino acid sequence of methionine-enkephalin is identical to that of the N-terminal portion of the C-fragment (.beta.-endorphin or .beta.-LPH[61-91]) of the peptide .beta.-lipotropin, which is found in large concentrations in the pituitary and in much lower concentrations in the brain. Other naturally-occurring fragments of .beta.-lipotropin are known, for example: .alpha.-endorphin (.beta.-LPH[61-76]) and .gamma.-endorphin (.beta.-LPH[61-77]). Both .beta.-lipotropin and the endorphins show morphine-like properties in various test systems, and it has been suggested that methionine-enkephalin is a breakdown product of the large opiate-like peptides. Enkephalin, its relationship to .beta.-lipotropin and the endorphins, and the pharmacological properties thereof, are reviewed in an article by Iversen et al., Nature, 262, 738 (1976). Recent developments are also described in detail in the "Proceedings of the International Narcotics Research Club Meeting, Aberdeen, U.K., July 19-22, 1976," published in OPIATES AND ENDOGENOUS OPIOID PEPTIDES, North Holland Publishing Company, Amsterdam, 1976.
Various structural variations of methionine-enkephalin and leucine-enkephalin are described in the literature. For example, the pentapeptide H-Tyr-Gly-Gly-Phe-Thr-OH, wherein the fifth amino acid residue (methionine or leucine) is replaced by threonine, is described by Chang et al., Life Sciences, 18, 1473 (1976).